POLYCHAETE DEPTH DISTRIBUTION IN A POSIDONIA-OCEANICA BED (RHIZOME AND MATTE STRATA) AND NEIGHBORING SOFT AND HARD BOTTOMS

The study was carried out in a Posidonia oceanica bed and in the neigh bouring sandy and rocky bottoms, at Ponza Island (Central Tyrrhenian S ea), in order to investigate the distributional pattern of the polycha ete community at different depths. A total of 15232 individuals of pol ychaetes belonging to 218 species was collected and three new species of Syllidae were found. The polychaete community of the matte can be c learly distinguished from the neighbouring communities; this is due to the high species richness in Posidonia, which is related to the heter ogeneity of the biotope. In the Posidonia bed the shallow stations (1 m and 5 m depth) were the richest in terms of species numbers and indi viduals due to the high shoot density of the seagrass bed. In the shal low waters polychaete community structure changed according to the dif ferent exposure to water movement. Rocky bottom species, which prefere ntially inhabit crevices, were more abundant in the matte exposed to w ater movement (5-10m depth). In contrast, soft-bottom species were mor e abundant in the sheltered zone (1 m depth) as well as in the deeper zone due to the reduction of the Posidonia bed.